Erosion resistant materials find use in many applications wherein surfaces are subject to eroding forces. For example, refinery process vessel walls and internals exposed to aggressive fluids containing hard, solid particles such as catalyst particles in various chemical and petroleum environments are subject to both erosion and corrosion. The protection of these vessels and internals against erosion and corrosion induced material degradation especially at high temperatures is a technological challenge. Refractory liners are used currently for components requiring protection against the most severe erosion and corrosion such as the inside walls of internal cyclones used to separate solid particles from fluid streams, for instance, the internal cyclones in fluid catalytic cracking units (FCCU) for separating catalyst particles from the process fluid. The state-of-the-art in erosion resistant materials is chemically bonded castable alumina refractories. These castable alumina refractories are applied to the surfaces in need of protection and upon heat curing hardens and adheres to the surface via metal-anchors or metal-reinforcements. It also readily bonds to other refractory surfaces. The typical chemical composition of one commercially available refractory is 80.0% Al2O3, 7.2% SiO2, 1.0% Fe2O3, 4.8% MgO/CaO, 4.5% P2O5 in wt %. The life span of the state-of-the-art refractory liners is significantly limited by excessive mechanical attrition of the liner from the high velocity solid particle impingement, mechanical cracking and spallation.
Ceramic-metal composites are called cermets. Cermets of adequate chemical stability suitably designed for high hardness and fracture toughness can provide an order of magnitude higher erosion resistance over refractory materials known in the art. Cermets generally comprise a ceramic phase and a binder phase and are commonly produced using powder metallurgy techniques where metal and ceramic powders are mixed, pressed and sintered at high temperatures to form dense compacts.
U.S. patent application Ser. No. 10/829,816 filed on Apr. 22, 2004 to Bangaru et al. discloses cermet compositions with improved erosion and corrosion resistance under high temperature conditions, and a method of making thereof. The improved cermet composition is represented by the formula (PQ)(RS) comprising: a ceramic phase (PQ) and binder phase (RS) wherein, P is at least one metal selected from the group consisting of Group IV, Group V, Group VI elements, Q is boride, R is selected from the group consisting of Fe, Ni, Co, Mn and mixtures thereof, and S comprises at least one element selected from Cr, Al, Si and Y. The ceramic phase disclosed is in the form of a monomodal grit distribution. U.S. patent application Ser. No. 10/829,816 is incorporated herein by reference in its entirety.
A need exists for cermet materials with high density, high fracture toughness and improved erosion and corrosion resistance properties for high temperature applications. The new and improved bimodal and multimodal cermet compositions of the instant invention satisfy this need. Furthermore, the present invention includes an improved method for protecting metal surfaces with bimodal or multimodal cermet compositions against erosion and corrosion under high temperature conditions.